This invention relates to a process for producing a fine powder of high-purity silk fibroin.
Powdered silk fibroin has heretofore been used mainly as an additive or base material for make-up cosmetics, because of its characteristic features such as moderate moisture absorption and retention properties, high affinity for the skin, excellent slip properties on the skin, good hydrophilic-lipophilic balance, and adequate ultraviolet absorption properties.
Currently available silk fibroin powders include, for example, (1) a powder of silk fibroin in fibrous form which is produced, as described in Japanese Patent Publication No. 24920/'75, by reducing silk thread directly to powder or by subjecting silk thread to a chemical treatment for its embrittlement and then reducing it to powder and (2) a powder of silk fibroin in granular form which is produced, as described in Japanese Patent Publication No. 4947/'51, by dissolving silk fibroin in a concentrated solution of a suitable neutral salt, dialyzing the resulting solution, spray-drying the colloidal solution so formed, and then grinding the resulting gel of silk fibroin.
The former powder of silk fibroin in fibrous form consists of filamentous fibers cut in short lengths rather than nearly globular particles, and the silk fibroin molecules contained therein are oriented in the direction of the fiber axis. Accordingly, when used as a base material for make-up cosmetics, this powder gives rise to various difficulties. For example, in mixing this powder with other ingredients in globular form, it is so liable to aggregation that a homogeneous final product is hardly obtained. Even if such a product is obtained, it shows poor slip properties upon application to the human skin and may occasionally produce round agglomerates of silk fibroin. Moreover, when used as an additive for cosmetic preparations, this powder has poor compatibility with other ingredients because of its high degree of molecular orientation, and has an inadequate moisture-controlling effect on the skin because of its poor ability to be swollen by water. Thus, it can be said that these difficulties prevent us from making good use of the excellent properties of silk fibroin.
Also in Japanese Patent Publication No. 1941/'64 is disclosed a process for producing silk fibroin suitable for use in chromatography. This process comprises dissolving silk fibroin in a cuprammonium solution or a solution of a copper complex (for example, a cupri-ethylenediamine solution), neutralizing the resulting solution with an acid, and then adding an alcohol to the neutralized solution to form a white precipitate of silk fibroin. As a result of confirmatory tests made by the present inventors, it has been found that this process requires a very large amount of alcohol and, moreover, the resulting precipitate is too sticky to be separated by filtration. Furthermore, the fine powder of silk fibroin produced by this process has such an abnormal degree of hydrophilic nature as to be soluble in hot water in an extreme case, and cannot be used as an additive for cosmetic preparations because of its adherance to the skin. Similarly, the aforesaid powder of silk fibroin produced by dissolving silk fibroin in a concentrated solution of a suitable neutral salt, dialyzing the resulting solution, and spray-drying the colloidal solution so formed is also abnormally hydrophilic and, therefore, unsuitable for use as an additive for cosmetic preparations.
In addition, there have been proposed other processes which involves hydrolyzing silk thread with an acid of alkali to prepare a silk fibroin solution and then precipitating the silk fibroin either by neutralization or by the addition of an alcohol. In the fine powders of silk fibroin produced by these processes, however, the molecular weight is reduced to those of oligomers and the characteristic properties of silk are completely lost.
X-ray diffraction analysis and infrared spectroscopic analysis have revealed that, in the conventional fine powders of silk fibroin produced by various processes involving the dissolution of silk fibroin, the silk fibroin molecules contained therein have either a random configuration or the .alpha.-configuration and the degree of crystallinity is so low as to imply the amorphous state rather than the crystalline state.
In order to overcome the above-described difficulties, the present inventors have made a series of intensive and extensive studies on the principle of rendering finely powdered silk fibroin hydrophobic to such a degree that it shows no stickiness in the presence of water, and have thereby completed this invention.